Add troubleshooting note to Praefect NTP clock sync check section

What does this MR do?

Adds a troubleshooting note to Praefect clock sync check section.

This information helps customers in air-gapped environments avoid issues with the clock sync check by showing how they can set an environment variable to direct Praefect to use internal NTP servers.

Related issues

Discovered during troubleshooting for ticket #349590

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